This invention relates to semiconductor memories and in particular to programable read-only memories.
Most programmable read-only memories (PROMS) currently being manufactured utilize metallic fusible links, or fuses, as the programming element in the memory array. These fuses laterally connect an array element, such as a diode, with a bit line. All of these components and/or their connections occupy space on the semiconductor chip area, such as the metal bit line, the laterally extending metal fuse, and the metal contact pad to the diode, for example. Since the memory array may contain thousands of array cells constituted by these components, such as 16,384 cells for a 16K PROM, the memory array occupies a major portion of the integrated circuit chip area for a high density state of the art PROM. Any space savings that can be achieved in the individual memory cells of the array will significantly reduce the size of the integrated chip and thereby increase the yield, that is, the number of good chips producible from a given wafer. As the yield increases, the unit manufacturing cost goes down.
One means of reducing the cell area would be to replace the lateral metal fuse with one that does not occupy any additional space than is required for the metal bit line and the contact to the diode. This could be done with a programmable element located directly over the diode. Some researchers have proposed vertical antifuses in the form of destructible diodes of reverse electrical orientation from the array diode. One such device of this type is disclosed in U.S. Pat. No. 3,641,516 to Castrucci et al. Programming of this type of antifuse is accomplished by subjecting the destructible diode to high reverse current flow and causing aluminum migration from the contact and bit line to short circuit the destructible diode.
It should be mentioned here that an antifuse is the opposite of a fuse link. An unprogrammed antifuse is non-conducting, and it is programmed by changing it to a conductive or low resistance state so as to close a circuit between the bit line and the array diode. A fuse link, on the other hand, is conductive when unprogrammed, and it is programmed by changing it to a non-conducting state so as to open or break the circuit between the bit line and the array diode.
Other proposals for an antifuse in memories have involved the use of deposited films of chalcogenide elements or amorphous silicon which become conducting when subjected to an electrical bias voltage. UK patent application No. GB 2086654A discloses deposited films of amorphous silicon and UK patent application No. GB 2065972A discloses deposited films of chalcogenide elements.